Kinetics and mechanism of the oxidation of phosphinic, phenylphosphinic, and phosphorous acids by pyridinium fluorotrioxochromate(VI)

Abstract

Oxidation of the lower phosphorus oxyacids by [C,H,NH] [CrO,F] results in the formation of corresponding oxyacids in the higher valence states. The reaction is first order with respect to the oxidant concentration. A Michaelis-Menten type kinetics was observed with respect to the substrate, indicating the formation of a complex in a pre-equilibrium. The formation constants and the rates of disproportionation of the complexes have been evaluated at different temperatures. The reaction exhibits a substantial primary kinetic isotope effect. The rates in 19 different organic solvents have been analysed using Kamlet-Taft and Swain equations. It has been found that the cation-solvating power of the solvents plays an important role. It is proposed that the 'inactive' tautomer of the phosphorus oxyacids is the reactive reductant, and that transfer of a hydride ion from the P-H bond to the oxidant in the rate-determining step occurs.The three lower oxyacids of phosphorus, viz. phosphinic (hypophosphorus) (1; R = H), phenylphosphinic (2; R = Ph), and phosphorous (3; R = OH), are known to exhibit tautomerism [reaction (l)]. 1,2 Generally, tautomer (A) is called the 'inactive' tautomer and (B) the 'active' one. The equilibrium constant, K,, has a value ca.Chromic acid4 oxidation of compound (2) has been reported to involve the 'active' form as the reactive species. In several oxidations, however, the participation of the 'inactive' form has been proposed, e.g. oxidation by permanganate and vanadium(v).6 Pyridinium fluorotrioxochromate(v1) is a versatile ~x i d a n t . ~We have previously reported its oxidation of alcohols and s ~l p h i d e s . ~ There seems to be no report on the oxidation of phosphorus compounds except on the synthetic aspects of oxidation of triphenylphosphine." In this paper we report the kinetics of oxidation of the three lower oxyacids of phosphorus by [C,H,NH][CrO3F] in dimethyl sulphoxide (dmso) as solvent. The mechanistic aspects are discussed.

Kinetics and Mechanism of the Oxidation of Oxyacids of Phosphorus by Benzimidazolium Dichromate

“…Further, the reported values [14,15] of the formation constant of chromic acid-PPA are 11 + 2 and 19 + 4 dm 3 mol À 1 . We have earlier obtained a value of 4.0 + 0.2 dm 3 mol À 1 for the formation constant of the PA -pyridinium fluorochromate complex [16]. Thus the very high value of the formation constant of the BIDC-phosphorus oxyacid complex is improbable, and it is highly unlikely that the form (B) is involved in the oxidation process.…”

Linear free energy relationships in the kinetic study of oxidation of phenols by quinolinium dichromate

“…The oxidation of arylmethyl sulphides by pyridinium dichromate [6] is found to proceed by a nonradical polar mechanism, while, when potassium dichromate [13] is used, the reaction follows a radical mechanism, involving cation radical as the reaction intermediate. The modes of oxidation of lower oxyacids of phosphorous by pyridinium fluorochromate [14], pyridinium chlorochromate [15], and pyridinium bromochromate [16] have been studied by Benerji and co-workers. The oxidation by fluorocompound involves formation of a complex between the oxidant and the substrate which decomposes in the slow step to give products.…”

Kinetics of the oxidation of phosphinic, phenylphosphinic, and phosphorous acids by N‐bromoacetamide

“…Except for oxidation of phosphorous acid by iodine [17] and that of phosphinic acid (hypophosphorus acid) by chloramine-T [MI, there seems to be no report of the oxidation by halogens or their compounds. In the chloramine-T oxidation [16], no attempt has been made to identify the reactive form of the tautomer. N-Halogeno compounds present halogens in the most effective oxidizing and electrophilic form.…”